In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location, airflow rates, and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles historically produce relatively high emissions due to the fact that the fuel and air burn essentially upon interaction, without mixing, and stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An alternate means of premixing fuel and air and obtaining lower emissions can occur by utilizing multiple combustion stages. In order to provide a combustor with multiple stages of combustion, the fuel and air, which mix and burn to form the hot combustion gases, must also be staged. By controlling the amount of fuel and air passing into the combustion system, available power as well as emissions can be controlled. Fuel can be staged through a series of valves within the fuel system or dedicated fuel circuits to specific fuel injectors. Air, however, can be more difficult to stage given the large quantity of air supplied by the engine compressor. In fact, because of the general design to gas turbine combustion systems, as shown by FIG. 1, air flow to a combustor is typically controlled by the size of the openings in the combustion liner itself, and is therefore not readily adjustable. An example of the prior art combustion system 100 is shown in cross section in FIG. 1. The combustion system 100 includes a flow sleeve 102 containing a combustion liner 104. A fuel injector 106 is secured to a casing 108 with the casing 108 encapsulating a radial mixer 110. Secured to the forward portion of the casing 108 is a cover 112 and pilot nozzle assembly 114.
However, while premixing fuel and air prior to combustion has been shown to help lower emissions, the amount of fuel-air premixture being injected has a tendency to vary due to a variety of combustor variables. As such, obstacles still remain with respect to controlling the amount of a fuel-air premixture being injected into a combustor.